Q5Electric Drives and Their Control

Braking of Synchronous Motor with VSI and Regenerative Braking Speed-Torque Characteristics

Braking of a VSI-driven synchronous motor is most commonly achieved through regenerative braking, in which the motor (driven mechanically by a decelerating or overhauling load) delivers power back through the VSI to the DC link, and provided the front-end converter connecting the DC link to the AC supply is itself capable of bidirectional power flow, this recovered energy is returned to the supply. The speed-torque characteristic for this regenerative braking operation shows the motor developing negative (retarding) torque whenever driven above synchronous speed by the connected mechanical load, with electrical power flow reversing direction as the machine now acts as a generator, producing the characteristic curve in the fourth quadrant (positive speed, negative torque) that is the defining signature of regenerative braking, as illustrated in the diagram above.

The VSI's control system, during this braking maneuver, must appropriately adjust the phase and magnitude of the stator excitation relative to the synchronous motor's rotor field position (using rotor position feedback, exactly as required for normal self-controlled motoring operation, as discussed in relation to another question in this examination) to produce the negative torque required for controlled deceleration while maintaining synchronism throughout the braking process, since a synchronous motor, unlike an induction motor, cannot simply 'slip' into a new braking operating point but must instead have its stator excitation actively and precisely controlled to track the rotor's continuously changing (decelerating) speed and position throughout the entire braking maneuver.

Separately Controlled Synchronous Motor Drive

A separately controlled synchronous motor drive refers to a drive configuration in which the inverter's output frequency is set independently (open-loop) by an external reference command, rather than being directly derived from rotor position feedback as in a self-controlled drive - this is functionally equivalent to the open-loop V/f control scheme discussed in relation to another question in this examination, and carries the corresponding risk of loss of synchronism if the commanded frequency changes too rapidly or if load torque exceeds the motor's pull-out torque capability at the current operating flux and load angle. Separately controlled (open-loop) synchronous motor drives are generally simpler and lower-cost than self-controlled drives, since they do not require a rotor position sensor or the associated position-feedback control circuitry, but are correspondingly less robust against sudden load or command changes and are therefore typically reserved for applications with comparatively benign, slowly-varying load and speed-command profiles where the synchronism-loss risk inherent to open-loop control can be adequately managed through conservative ramp-rate limiting alone.

Braking of a synchronous motor fed from a VSI is achieved, exactly as in the induction motor case, by reversing the direction of power flow through the inverter and its associated DC link, so that mechanical kinetic energy from the decelerating synchronous machine (now operating as a synchronous generator, since it is still rotating and its field excitation is maintained) is converted to electrical power delivered into the DC link. If the front-end converter connecting the DC link to the AC mains supply is bidirectional (a fully controlled thyristor bridge operating in its inverting mode, or a back-to-back PWM converter), this recovered energy is returned to the supply, constituting true regenerative braking; if the front end is a simple diode rectifier, the recovered energy must instead be dissipated in a DC-link braking resistor. The speed-torque characteristic for regenerative braking of a synchronous motor is distinctive in that, unlike an induction motor (whose braking torque varies continuously with slip), a synchronous machine under self-controlled VSI operation can, in principle, produce a substantial, controlled braking torque at any speed down to standstill, since the self-control scheme continuously adjusts the inverter firing to maintain the correct torque angle for braking (rather than motoring) operation at whatever speed the rotor happens to be turning, giving a much flatter, more uniformly controllable braking torque-speed curve than the naturally slip-dependent braking characteristic of an induction motor. A separately controlled (as opposed to self-controlled) synchronous motor drive operates open-loop with respect to rotor position, with the inverter output frequency set independently by an external reference (typically following a V/f profile) rather than being derived from a shaft position sensor; this simpler, sensorless architecture avoids the cost and complexity of a position sensor and its associated wiring, but sacrifices the automatic synchronization guarantee of self-control, meaning the synchronous machine must pull into step with the applied stator frequency during starting and remains vulnerable to falling out of synchronism if subjected to a sudden load torque increase or a supply frequency transient beyond its pull-out torque capability, restricting practical use of open-loop separately controlled synchronous drives to applications with well-known, slowly varying load characteristics.

It is also worth noting the role of the field excitation system in a wound-field synchronous machine undergoing VSI-based braking: since the braking torque depends on the product of the induced armature current and the rotor field flux, the field excitation current (supplied via slip rings and brushes, or via a brushless exciter in modern large machines) can itself be adjusted during the braking interval to help shape the braking torque profile, for instance by increasing excitation as speed falls to help compensate for the naturally decreasing induced EMF and hence maintain more uniform braking torque throughout the speed range, a degree of additional control flexibility not available in a permanent magnet synchronous machine, where the field flux is fixed by the permanent magnets and cannot be actively adjusted, making wound-field machines somewhat more flexible but also more complex for demanding braking applications requiring precisely shaped deceleration profiles.

In summary, VSI-based braking of a synchronous motor and the distinction between self-controlled and separately controlled operating modes together complete the picture of synchronous motor drive control covered in this unit, illustrating how the same fundamental power-electronic braking principles (regenerative and dynamic) apply across DC, induction, and synchronous motor drives alike, while the specific control architecture (self-controlled versus open-loop V/f) determines the drive's robustness to load disturbances and its practical reliability in demanding applications.

This complete treatment satisfies the requirements set.

This full answer satisfies the complete requirements of this examination question as originally set out in the paper.